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A lens is one or more pieces of optical material (most commonly glass or plastic) designed to collect and focus all wavelengths of light at a single focal point to form a sharp image on the film.
When you look at the specifications of a camera you will notice that its lens is made of elements and groups.
The term element refers to the individual
pieces of glass. The term group can refer to a single element, or two or more elements
So a lens that has 5 elements in 4 groups has 5 pieces of glass with 2 cemented together.
The use of multiple elements allows more optical aberrations to be corrected than is possible with a single element, but by itself the number of elements is no guarantee of quality.